Quadrature modulator with feedback control and optical communications system using the same

ABSTRACT

The method and system are disclosed for automatic feedback control of integrated optical quadrature modulator for generation of optical quaternary phase-shift-keyed signal in coherent optical communications. The method comprises the steps of detecting at least a part of an output optical signal from the QPSK modulator, extracting of a particular portion of the output signal in frequency domain, and processing the signal in frequency domain to optimize the transmission of an optical link. The system and method of optical communications in fiber or free space are disclosed that implement the quadrature data modulator with automatic feedback control.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims is a continuation of U.S. Ser. No.11/679,376 filed Feb. 27, 2007, which is a continuation-in-part of U.S.Ser. No. 10/613,772 filed Jul. 2, 2003 and also a continuation-in-partof U.S. patent applications Ser. No. 10/669,130 filed on Sep. 22, 2003and Ser. No. 10/672,372 filed on Feb. 7, 200, all of them incorporatedherein by reference.

FIELD OF INVENTION

This invention relates generally to optical communications especiallycoherent communication with quaternary phase shift keying (QPSK)modulation format. Quadrature modulators are used in these systems forQPSK data encoding. The present invention relates to methods and systemsof control for integrated quadrature modulators.

BACKGROUND OF THE INVENTION

Multi-level phase-shift-keying (PSK) offers high spectral efficiencytransmission in coherent optical communication systems. Quaternary PSK(QPSK) format, in particular, has recently received much attention. Anoptical QPSK signal can be generated, e.g., by an integrated LiNbO₃quadrature modulator (QM) with two parallel Mach-Zehnder modulators(MZMs) nested in a MZ interferometer. Each MZM is driven to produce abinary PSK (BPSK) signal. An optical QPSK signal is produced when twoMZMs are biased at their null transmission points and the MZinterferometer is biased at the quadrature phase (π/2). FIG. 1 shows aschematic of a quadrature modulator 1 known in the prior art. Theprinciple of its operation is as follows. Input optical beam 2 issplitted into two arms of the MZ interferometer by a splitter 3. TwoMach-Zehnder modulators 4 and 5 are placed in parallel; each MZM beinglocated in each arm of the MZ interferometer. The biases of the MZMs arecontrolled by control signals 7 and 8 and driven by RF data signals 8and 9. The Phase port of the QM 10 controls relative phase shift betweenthe arms of the MZ interferometer.

In modern communication systems operating at a speed exceeding 10Gbits/s, a precise stabilization of QPSK modulators is required. Thereis a need for an automatic feedback control loop that searches for thesebiases and phase operating points of the QM at initial startup andmaintains them during operation.

SUMMARY OF THE INVENTION

The method and system are disclosed for an automatic feedback control ofintegrated quadrature modulator for generation of optical quaternaryphase-shift-keyed signal in coherent optical communications.

The method comprises the steps of detecting at least a part of outputsignal from the modulator; extracting of a particular portion of theoutput signal in RF frequency domain; and minimizing the output signalin RF frequency domain by dithering a voltage applied to a phase shifterof the QPSK modulator. Additionally the method includes detecting theoutput signal power and minimizing this output signal power by ditheringa voltage applied to a first and a second bias of the QPSK modulator.

Alternative method includes detecting the output signal power andmaximizing this output signal power by dithering a voltage applied to afirst and a second bias of the QPSK modulator.

The control loop algorithm uses a steepest decent algorithm to searchfor optimal operating points of the quadrature modulator via ditheringof its biases and phase. The criteria, for the dithering are based onminimization of the RF signal voltage and maximization or minimizationof the optical average power of the output signal.

An optical communications system is proposed that incorporates QPSKmodulator for data encoding with the feedback loop control of themodulator to improve transmission performance. In the preferredembodiment the communication system includes an integrated coherentreceiver based on 90-degrees optical hybrid

Yet another object of the present invention is an optical communicationssystem operating in two polarization states of light The systemincorporates two QPSK modulators each having its feedback loop control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a quadrature modulator with two parallelMZMs nested in a MZ interferometer with a phase bias (Prior Art).

FIG. 2 shows a schematic diagram of control unit for quadraturemodulator in optical communications.

FIG. 3 shows simulation results for (a) average power versus biasvoltage of MZM and (b) integrated RF spectral power versus Δφ_(IQ) forNRZ drive signal swing of 0.75 and 1.2V_(π).

FIG. 4 shows constellation plots of the QM optical output at startup (a)and after 50 iterations of the control loop (b). Plot of deviations ofthe two biases and phase from then optimal points (π and π/2) versusiteration number are shown in (c).

FIG. 5 shows BER versus received optical power of the differentiallydetected QPSK signal with automatic control loop or with manualadjustment of the QM. Inset shows eye diagram (top) of thedifferentially detected 12.5 GSmp/s QPSK signal and the directlydetected output from the QM (bottom), Horizontal scale: 20 ps/div.

FIG. 6 shows block diagram of an optical communication system thatincludes quadrature modulator having a control unit according to FIG. 2.

FIG. 7 shows a block diagram of a coherent optical receiver for theoptical communications system of FIG. 6.

FIG. 8 shows a block diagram of a coherent optical receiver operating intwo polarization states of light.

FIG. 9 shows block diagram of an optical communication system operatingin two polarization states of light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A schematic diagram for a feedback control loop for the quadraturemodulator is shown in FIG. 2. A light source 12 is launched into aquadrature modulator 1. In the preferred embodiment the light source isa CW or pulsed laser. The quadrature modulator is driven by twohigh-speed (>10 Gbits/s) non-return-to-zero (NRZ) binary data streamsthrough RF ports 8 and 9. The optical output of the quadrature modulatoris divided into two paths using a tap 13. The optical beam 14 from thetapped output is impinged to a low-speed (e.g., 750 MHz) photodetector15. In the preferred embodiment the photodetector 15 is a single photonabsorption photodetector. The electrical signal 16 from thephotodetector 15 is divided into two by splitter 17 with one pathconnected to a DC block 18 to reject dc components of the electricalsignal. This is followed by a RF spectral power detector (e.g., Schottkydiode) 19 to extract the low-frequency RF spectral power (V_(RF)). Thesignal is then digitized using an analog-to-digital converter (ADC) 20connected to a digital signal processing (DSP) unit 21. The DSP unitcontains a code that executes the control loop algorithm. The secondpath from the photodetector output is directly connected to another ADC22 that provides monitoring of the optical average power. Outputs of theDSP unit are converted in analog signals by digital-to-analog converters23, 24, 25 and directed to the two bias ports 6 and 7 and the phase port10 of the quadrature modulator 1.

The principle of feedback loop operation becomes clear from thefollowing detailed description of its operation. FIG. 1 shows aschematic of a QM with two push-pull type MZMs with RF and DC biaselectrodes nested in a MZ interferometer with a phase electrode forquadrature bias. Consider a single MZM, the directly detected opticaloutput power is P_(o)(t)=(kP_(i)2){1+cos [π(V_(s)(t)+V_(B))/V_(π) },where V_(s)(t) is the NRZ drive signal with a peak-to-peak voltage swingV_(pp), V_(B) is the bias voltage, V_(π) is the half-wave voltage, P_(t)is the input optical power, and k accounts for the insertion loss of theMZM. To generate optical BPSK signal, the MZM bias is set to the nulltransmission with V_(B)=±V_(π), ±3V_(π), . . . , and V_(s) variesbetween ±V_(π). The output average power over a period of time T is

${\langle{P_{o}(t)}\rangle} = {\frac{{kP}_{i}}{2}{\left( {1 + {\frac{1}{T}{\int_{0}^{T}{{\cos \left\lbrack {\frac{\pi}{V_{\pi}}\left( {{V_{s}(t)} + V_{B}} \right)} \right\rbrack}\ {t}}}}} \right).}}$

Taking the derivative of the above with respect to V_(B) and equating tozero gives

$\begin{matrix}{\frac{\partial{\langle{P_{o}(t)}\rangle}}{\partial V_{B}} = {{- \frac{k\; P_{i}}{2}}\frac{\pi}{V_{\pi}}\frac{1}{T}{\int_{0}^{T}{{\sin \left\lbrack {\frac{\pi}{V_{\pi}}\left( {{V_{s}(t)} + V_{B}} \right)} \right\rbrack}\ {t}}}}} \\{{= \left. 0\Rightarrow{\sin \left\lbrack {\frac{\pi}{V_{\pi}}\left( {{V_{s}(t)} + V_{B}} \right)} \right\rbrack} \right.}\ } \\{= 0.}\end{matrix}$

The above is satisfied if T_(s)=mV_(π) and V_(B)=nV_(π)(m,n=0,±1,±2, . .. ). Taking the second derivative of

P_(o)(t)

with respect to V_(B) gives

$\frac{\partial^{2}{\langle{P_{o}(t)}\rangle}}{\partial V_{B}^{2}} = {{- \frac{k\; P_{i}}{2}}\left( \frac{\pi}{V_{\pi}} \right)^{2}\frac{1}{T}{\int_{0}^{T}{{\cos \left\lbrack {\frac{\pi}{V_{\pi}}\left( {{V_{s}(t)} + V_{B}} \right)} \right\rbrack}\ {{t}.}}}}$

Therefore, the conditions for extrema of the average optical power are

$\left. {\langle{P_{o}(t)}\rangle}_{\max}\Rightarrow{\frac{\partial^{2}{\langle{P_{o}(t)}\rangle}}{\partial V_{B}^{2}} < 0}\Rightarrow{{\cos \left\lbrack {\frac{\pi}{V_{\pi}}\left( {{V_{s}(t)} + V_{B}} \right)} \right\rbrack}\  > 0} \right.,\left. {\langle{P_{o}(t)}\rangle}_{\min}\Rightarrow{\frac{\partial^{2}{\langle{P_{o}(t)}\rangle}}{\partial V_{B}^{2}} > 0}\Rightarrow{{\cos \left\lbrack {\frac{\pi}{V_{\pi}}\left( {{V_{s}(t)} + V_{B}} \right)} \right\rbrack}\  < 0.} \right.$

For null transmission of the MZM, V_(B)=≧V_(π), ±3V_(π), . . . , so thatthe above can be written as follows

$\left. {\langle{P_{o}(t)}\rangle}_{\max}\Rightarrow{\frac{\partial^{2}{\langle{P_{o}(t)}\rangle}}{\partial V_{B}^{2}} < 0}\Rightarrow{{\cos \left\lbrack {\frac{\pi}{V_{\pi}}{V_{s}(t)}} \right\rbrack}\  < 0}\Rightarrow{V_{\pi} < V_{pp} \leq {2V_{\pi}}} \right.,\left. {\langle{P_{o}(t)}\rangle}_{\min}\Rightarrow{\frac{\partial^{2}{\langle{P_{o}(t)}\rangle}}{\partial V_{B}^{2}} > 0}\Rightarrow{{\cos \left\lbrack {\frac{\pi}{V_{\pi}}{V_{s}(t)}} \right\rbrack}\  > 0}\Rightarrow{0 < V_{pp} < {V_{\pi}.}} \right.$

Therefore, in order to maintain null transmission of the MZM for BPSKoperation the average power of the MZM output should be maximized forV_(π)<V_(pp)≦2V_(π) or minimized for 0<V_(pp)<V_(π).

FIG. 3 a shows a simulated output optical average power of the QM versusV_(B) for V_(pp) of 075 and 1.2V_(π). The simulation uses a 12.5 Gb/sNRZ pseudo-random binary sequence (PRBS) with a word length of 2¹¹−1with realistic waveforms (finite rise and fall times and ringings)driving the two MZMs of the QM biased to quadrature phase. The two NRZsignals are complementary with a 2-symbol relative time delay. Gaussiannoise was added to the chive signal and to the input optical field tocheck the robustness of the response. As can be seen, the simulationresult is consistent with the dependence of the average power on the MZMbias analyzed above.

Consider now the phase bias of the MZ interferometer of the QM where thephase shift between the two BPSK signals (I and Q) is Δφ_(IQ). It can beshown that the directly detected output power of the QM is given by

P _(QM)=(kP _(i)/4){1−cos(πV _(I) /V _(π))/2−cos(πV _(Q) /V _(π))/2+2sin[πV_(I)/(2V _(π))] sin [πV _(Q)/(2V _(π))] cos(Δφ_(IQ))},

where V_(I) and V_(Q) are the NRZ binary data signals applied to the twoMinis biased at their null transmission points (V_(B)=V_(π)). AssumingV_(I) and V_(Q) varies between ±V_(π), the detected output can thus besimplified as follows

$P_{QM} = \left\{ \begin{matrix}{{k\; {{P_{i}\left\lbrack {1 + {\cos \left( {\Delta\varphi}_{IQ} \right)}} \right\rbrack}/2}},} & {{{{for}\mspace{14mu} V_{I}} = {V_{Q} = {\pm V_{\pi}}}},} \\{{k\; {{P_{i}\left\lbrack {1 - {\cos \left( {\Delta\varphi}_{IQ} \right)}} \right\rbrack}/2}},} & {{{for}\mspace{14mu} V_{I}} = {{{\pm V_{\pi}}\mspace{14mu} {and}\mspace{14mu} V_{Q}} = {\mp {V_{\pi}.}}}}\end{matrix} \right.$

It is clear that data-like binary pattern will appear at the output ofthe QM if the MZ interferometer is not in quadrature (Δφ_(IQ)≠π/2). TheRF spectrum of P_(QM) contains low-frequency components due to this datapattern. Therefore, a minimum integrated RF spectral power of P_(QM)should be an indication that Δφ_(IQ) is close to π/2. FIG. 3 b shows thesimulated integrated RF spectral power of P_(QM) (V_(RF)) for V_(pp) of0.75 and 1.2V_(π) versus Δφ_(IQ) using similar NRZ drive signals withGaussian noise as in FIG. 3 a. The results are in agreement with theanalysis. Note that the dependence of V_(RF) on Δφ_(IQ) is not affectedby V_(pp). Based on the analysis and results shown in FIG. 3, a QMcontrol loop algorithm and model was developed.

The control loop uses a steepest decent algorithm to search for optimaloperating points of the QM via dithering of its biases and phase. Thedithering is performed continuously while monitoring the two feedbacksignals: V_(RF) and the average optical power. The criteria for thedithering is based on minimization of the signal V_(RF) and maximizes orminimizes the optical average power if the peak-to-peak NRZ drive signalis above or below the half-wave voltage (V_(π)) of the quadraturemodulator as described earlier.

FIG. 4 shows typical simulation results of the control loop withGaussian noise added to the drive signals and to the input optical fieldas before. One can see that the control loop is quite robust even in thepresence of significant amount of amplitude and phase noise. Convergenceto optimal operating points was observed for many random initial biasesand phases of the QM tested.

EXAMPLE 1 QM Control Loop Experiment

An experiment on closed-loop control of the QM was conducted toinvestigate its performance for generation of a 12.5-GSym/s optical QPSKsignal. A packaged LiNbO₃ QM was driven by two 12.5 Gb/s NRZ PRBS (wordlength: 2¹⁵−1) signals. The two NRZ signals are complementary with a2-symbol relative time delay. The NRZ drive voltage swing applied to theQM was V_(PP)˜1.2V_(π). The output of the QM was tapped off and directedto a 750-MHz photodetector where its output was divided into two withone path connected to a Schottky diode detector to extract thelow-frequency RF spectral power (V_(RF)). The signal was amplified anddirected to a commercial off-the-shelf (COTS) analog-to-digitalconverter (ADC) connected to a desktop computer (PC) running a codebased on the control loop algorithm described earlier. The second pathwas amplified and directly connected to the ADC that provides monitoringof the optical average power. Outputs of a COTS digital-to-analogconverter connected to the PC are directed to the two MZM bias ports andthe phase port of the QM. This completes the QM feedback control loop.

The 12.5 GSym/s optical QPSK signal was directed to a receiver with anoptical pre-amp and a band-pass filter. Differential detection of the12.5 GSym/s QPSK signal was employed using a fiber-based asymmetricMach-Zehnder (AMZ) interferometer with a one-symbol differential delay(80 ps). The two outputs of the AMZ demodulator were directed to a15-GHz balanced photoreceiver. The differential phase shift of the AMZwas adjusted to approximately ±π/4 to obtain maximum eye opening. FIG. 5shows BER measurements of the differentially detected QPSK signal usingthe automatic control loop. Measurements using manual adjustment of theDC biases and phase of the QM by minimizing the BER are also shown. Apower penalty of about 1 dB at 10⁻⁹ BER was observed for the controlloop. This is attributed to the dithering and the relatively flatresponses of V_(RV) and the average power near their optimal points ascan be seen in FIG. 2. Nevertheless, the control loop concept wasdemonstrated and validated using COTS components. The QM control loopwas operated continuously for about 20 hours with no degradation inperformance. The control loop is expected to work for higher symbolrates since no high-speed components are required in the loop. Thecontrol loop also works for RZ format of the QPSK signal. Generation anddetection of 12.5 GSym/s RZ-DQPSK has using the QM control loop wasconducted and its performance was verified.

Coherent communications system with quadrature modulator having acontrol unit is another object of the present invention.

The block diagram of a coherent communications system according to thepresent invention is shown in FIG. 6. Optical transmitter comprises alight source 12 and a quadrature modulator 1 with a control unit 26 thatallows optimizing the data transmission performance. Encoded opticalsignal is transmitted over the transmission link 28 to the coherentoptical receiver 29 where the data is decoded by mixing the transmittedoptical signal with a signal from a local oscillator 30.

In the preferred embodiment the coherent optical receiver is anintegrated receiver based on 90-degrees optical hybrid as disclosed inco-pending U.S. patent applications Ser. No. 10/669,130 filed on Sep.22, 2003 and Ser. No. 10/672,372 filed on Feb. 7, 2007 by the sameinventors, incorporated herein by references.

FIG. 7 illustrates a coherent receiver 29 of the preferred embodiment.It includes an optical interface 31 and a receiving unit 32. Theinterface includes a first device input 33 and a second device input 34;first 35, second 36, third 37 and fourth 38 couplers (mixers); a firstphase shifter 39 and a second phase shifter 40, and first 41, second 42,third 43, and fourth 44 outputs. The optical interface further includestwo crossing waveguides 45 and 46, which cross each other. The receivingunit 32 includes four photodetectors 47, 48, 49, and 50 having outputs51, 52, 53, and 54 respectively. The receiver further includes datadigital signal processing unit 55.

The first 33 and the second 34 device inputs both are connected,respectively, to the first coupler 35 and the second coupler 36. Oneoutput of the first coupler 35 is connected to one input, of the thirdcoupler 37 while another output of the first coupler 35 is connected tothe one input of the fourth coupler 38 by a first crossing waveguide 45.An output of the second coupler 36 is connected to another input of thefourth coupler 38 while another output of the second coupler 36 isconnected to another input of the third coupler 37 by a second crossingwaveguide 46. The optical interface also includes at least one phaseshifter positioned between two locations. The first location is one ofthe outputs of the first or second coupler. The other location is one ofthe inputs of the third or fourth couplers, which corresponds (connectedby a crossing waveguide) to the first location. The first and secondoutputs of the third coupler 37 produce the first 41 and the second 42device outputs, respectively. The first and second outputs of the fourthcoupler 38 produce the third 43 and the fourth 44 device outputs,respectively.

Signals coming out of the outputs 41, 42, 43, and 44 impingephotodetectors 47, 48, 49, and 50, respectively. It is preferred thatthe photodetectors are PIN photodiodes. The photodiodes are located atequal distance apart. The distance between the neighbor photodiodes canbe from 0.01 to 1 mm. In the preferred embodiment the distance is from0.1 to 0.2 mm. The array of the photodiodes is fabricated on top of asingle substrate. InGaAs photodiodes produced by OSI Optoelectronics,Inc. (Hawthorne, Calif.) are examples of such photodiodes. In thepreferred embodiment the substrate is made of alumina.

In another embodiment an optical signal in two polarization states istransmitted over the communications link and by a two polarizationcoherent detector. One embodiment of a coherent optical receiver 59operating in two polarizations is shown in FIG. 8. It includes anoptical interface 60 and a set of photodiodes 61. The interface includesa first device input 62, a second device input 63, a third device input64, a fourth device input 65, a fifth device input 66; a polarizationbeam splitter 67, first 68, second 69, third 70, fourth 71, fifth 72,sixth 73, seventh 74, and eighth 75 couplers (mixers); a first phaseshifter 76, a second phase shifter 77, a third phase shifter 78, and afourth phase shifter 79, first 80, second 81, third 82, fourth 63, fifth84, sixth 85, seventh 86, eighth 87, ninth 88, and tenth 89 deviceoutputs. The device further includes two sets of crossing waveguides (90and 91) and (92 and 93). The receiver may optionally include twoalignment waveguides 94 and 95 are located on opposite sides of theoptical interface 60.

Signals coming out of the ten outputs 80, 81, 82, 83, 84, 85, 86, 87,88, 89 impinge photodetectors 96, 97, 98, 99, 100,101, 102, 103, 104,105, respectively. It is preferred that the photodetectors are PINphotodiodes. Similarly to the device in FIG. 7, the photodiodes arelocated at equal distance apart. The distance between the neighborphotodiodes can be from 0.01 to 1 mm. In the preferred embodiment thedistance is from 0.1 to 0.2 mm. During the fabrication the opticalinterface 60 alignment relatively the photodetector unit 61 is performedby light passing through waveguides 94 and 95 and positioning the unit61 to maximize the current from photodiodes 96 and 105. The accuracy ofalignment is at least 1 micron. In the preferred embodiment the accuracyis about 0.1 micron.

An optical communication link shown in FIG. 9 illustrates lighttransmission in two polarizations according to the present invention.Two-polarization transmitter 106 includes the light source 12 and two(PSK modulators 1 a and 1 b, each being controlled by its control unit26 a and 26 b respectively. Output beam from the light source 12 issplit into two beams by polarization beam splitter 107. Each ofmodulators 1 a and 1 b operates with the light of one polarizationstate. Output beam from the modulators 1 a and 1 b being combined by apolarization beam combiner 108 is transmitted over the communicationslink 28 towards the coherent optical receiver 59 operating in two lightpolarization states The transmitted signal impinges the input 64 of thereceiver 59. In the receiver 59 the transmitted signal is mixed with twosignals from a local oscillator 109 operating in two light polarizationstates. Local oscillator beams 110 and 110 have orthogonal polarizationstates and impinge the receiver 59 via the inputs 63 and 65 shown inFIG. 8.

The elements in the optical receivers 31 and 60 can each be formed aspart of a single planar chip made of an electro-optical material. Invarious embodiments, the chip is a monolithic piece of a wafer that canbe made of semiconductor or ferroelectric materials including but notlimited to LiNbO₃, and the like. In various embodiments, differenteffects relative to the output of the chip of the present invention arepossible., including but not limited to, (i) thermo-optical, (ii)electro-optical, (iii) electro-absorption, and the like. Theelectro-optical material, which can be LiNbO₃, can be cut at X, Y, or Zplanes. The device of the present invention can utilize a variety ofdifferent processes in its creation, including but not limited to, metalin-diffusion and/or (annealed) protonic-exchange technology, wetetching, reactive ion (beam) etching, plasma etching, and the like.

Integration of components in a single chip, such as LiNbO₃ and the like,can, among other things, reduce cost, improve performance, and providebetter stability and control. The optical interfaces 31 and 60 of thepresent invention, when integrated on a single chip and/or in singlepackage, can be used for various applications, including those thatrequire simultaneous measurement of phase and amplitude of the opticalfield. In the preferred embodiment the receiving units 32 and 61 includethe balanced receivers and optionally Trans-Impedance Amplifiers (TIAs),all formed as a part of a single integrated package.

Alternatively the integrated device chip can be made of thesemiconductor material selected from Si and InP.

The description of the invention has been presented for purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Obviously, manymodifications and variations will be apparent to practitioners skilledin this art. The application of the disclosed quadrature modulator isnot limited to optical communications either free-space, fiber orwaveguide. The present invention is related to any other possibleapplications of QPSK modulation technique.

1. The method for automatic feedback control of an integrated opticalquadrature modulator comprising the steps of: data encoding of an inputoptical signal in the quadrature modulator, detecting at least a firstpart of an output optical signal from the quadrature modulator,extracting of a particular portion of the output signal in RF frequencydomain and minimizing the portion of the output signal in RF frequencydomain by tuning a voltage applied to a phase shifter connected to I andQ components of the quadrature modulator, the phase shifter providing a90 degrees phase shift between I and Q components of the optical signal;achieving convergence to an optimal operating point of the quadraturemodulator by the voltage tuning; and outputting a second part of theoutput optical signal with 90-degrees phase shift between I and Q signalcomponents, wherein the second part of the output optical signal is PSKmodulated.
 2. The method of claim 1, wherein the tuning consists ofdithering a control voltage applied to the phase shifter.
 3. The methodof claim 1, wherein the tuning is performed using a steepest descentalgorithm.
 4. The method of claim 1, wherein the tuning is performedcontinuously during the quadrature modulator operation.
 5. The method ofclaim 1, further comprising discontinuing tuning when the optimaloperating point is achieved.
 6. The method of claim 1, wherein Icomponent of the output signal is produced a first Mach-Zehnder (MZM)modulator and Q component of the output signal is produced a secondMach-Zehnder modulator, two MZMs forming a MZ interferometer, whichserves as the quadrature modulator; and wherein each MZM is driven toproduce binary PSK.
 7. The method of claim 6, wherein the optimaloperating point corresponds to each of the MZMs is biased at their nulltransmission point and the MZ interferometer is biased at the quadraturephase.
 8. The method of claim 1, wherein the second output opticalsignal is QPSK modulated.
 9. The method of claim 1, wherein the inputsignal to the quadrature modulator is pulsed signal.
 10. The method ofclaim 1, further comprising: detecting a power of the output opticalsignal, tuning a first voltage applied to a first bias of the quadraturemodulator and a second voltage applied to a second bias, and minimizingthe power of the output optical signal.
 11. The method of claim 1,further comprising: detecting a power of the output optical signal,tuning a first voltage applied to a first bias of the quadraturemodulator and a second voltage applied to a second bias, and maximizingthe power of the output optical signal.
 12. A quadrature modulator,comprising: a first modulator producing I component of an optical beam;a second modulator producing Q component of the optical beam; the secondmodulator positioned in parallel to the first modulator forming a MZinterferometer; a phase shifter introducing a 90-degrees phase shiftbetween I and Q components of the optical beam; a combiner combining Iand Q components of the optical beam and outputting a quadraturemodulated optical signal; and a feedback loop connected to the combinerand the phase shifter; the feedback loop optimizing operation of thephase shifter to achieve the modulator operation at an optimal operatingpoint.
 13. The modulator of claim 12, wherein the first and the secondmodulators are Mach-Zehnder modulators.
 14. The modulator of claim 13,wherein the feedback loop further includes branches providingoptimization of MZMs operation.
 15. The modulator of claim 13, whereinthe optimal operation point corresponds to each of the MZMs is biased attheir null transmission point and the MZ interferometer is biased at aπ/2 phase.
 16. The modulator of claim 12, wherein the feedback loopcomprises a waveguide connected to the quadrature modulator output,extracting of a particular portion of the output signal, converting inRF frequency domain and minimizing the portion of the output signal inRF frequency domain by tuning a voltage applied to a phase shifter ofthe MZ interferometer.
 17. The modulator of claim 16, further comprisinga DSP unit controlling the feedback loop.
 18. An optical communicationssystem, comprising: a transmitter for creating and sending a quadraturemodulated signal, the transmitter including a quadrature modulatorproducing QPSK data modulation of an optical signal; the quadraturemodulator comprising a first feedback loop connected to an output of thequadrature modulator and controlling a phase shifter of the quadraturemodulator to achieve its operation at 90 degrees point; a receiver,receiving the quadrature modulated signal and recovering QPSK modulateddata.
 19. The system of claim 18, wherein the first feedback loopprovides an improved system performance allowing to achieve higher BERthan without the feedback loop.
 20. The system of claim 18, furthercomprising a second and a thirds feedback loop connected to an output ofthe quadrature modulator and controlling a first and a secondMach-Zehnder modulators, which produce I and Q components of the QPSKsignal, respectively; and all three feedback loops improving performanceof the system allowing to achieve higher BER than without the feedbackloops.